Chocolate pound cake from a box mix?

I'm making a birthday cake for my boyfriend. A pound cake is unorthodox but hey, whatever. Because I didn't know if I had all the ingredients, I bought a box mix. He'd like the pound cake now to be chocolate. Help!

If all you want is color and a hint of taste, replacing a tablespoon or so of flour with cocoa might do it... but I really don't know what a "box mix" entails here. (The classic "pound cake" is pretty darn simple ingredientwise: flour, butter, eggs, sugar in equal weights. Why a mix?)

Really adding *chocolate* to any kind of cake requires substantially rebalancing the recipe.